Constant velocity universal joints to be used as, for example, means for transmitting a rotational force from an engine of an automobile to wheels at constant velocity are classified into such two types as a fixed type constant velocity universal joint and a plunging type constant velocity universal joint. The two types of constant velocity universal joints have structure capable of transmitting rotational torque at constant velocity even when two shafts, namely, a drive shaft and a driven shaft are coupled to each other to form an operating angle.
A drive shaft configured to transmit power from an engine of an automobile to a driving wheel needs to cope with angular displacement and axial displacement due to a change in relative positional relationship between the engine and the wheel. Therefore, in general, a plunging type constant velocity universal joint configured to allow both the angular displacement and the axial displacement is mounted on an engine side (inboard side), and a fixed type constant velocity universal joint configured to allow only the angular displacement is mounted to a driving wheel side (outboard side). The both constant velocity universal joints are connected to each other with a shaft.
As illustrated in FIG. 15, the fixed type constant velocity universal joint includes an outer joint member 123, an inner joint member 126, a plurality of balls 127, and a cage 128. The outer joint member 123 has a plurality of track grooves 122 formed in a radially inner surface 121 at equal intervals in a circumferential direction and along an axial direction. The inner joint member 126 has a plurality of track grooves 125 formed in a radially outer surface 124 at equal intervals in a circumferential direction and along the axial direction to be paired with the track grooves 122 of the outer joint member 123. The plurality of balls 127 are interposed between the track grooves 122 of the outer joint member 123 and the track grooves 125 of the inner joint member 126 and configured to transmit torque. The cage 128 is interposed between the radially inner surface 121 of the outer joint member 123 and the radially outer surface 124 of the inner joint member 126 and configured to retain the balls 127.
Further, a female spline 129 is formed in a radially inner surface of a shaft hole of the inner joint member 126, and a male spline 131 is formed in an end portion of a shaft 130 to be fitted into the shaft hole of the inner joint member 126. Thus, when the end portion of the shaft 130 is fitted into the shaft hole of the inner joint member 126, the female spline 129 of the inner joint member 126 and the male spline 131 of the shaft 130 are fitted so that torque can be transmitted. A circlip 132 is mounted to the end portion of the shaft 130. With this, the shaft 130 is prevented from being coming off. The outer joint member 123 includes a mouth section 123a having the plurality of track grooves 122 formed in the radially inner surface 121 and a stem section (shaft section) 123b projecting from a bottom wall of the mouth section 123a. 
As illustrated in FIG. 16, the plunging type constant velocity universal joint includes an outer joint member 141, a tripod member 143, and rollers 144. The outer joint member 141 has three track grooves 140 being formed along an inner periphery thereof and extending in an axial direction and has roller guide surfaces 140a respectively opposed to inner walls of the track grooves 140. The tripod member 143 has three journal 142 and serves as an inner joint member. The rollers 144 are rotatably supported by the journal 142 and rollably placed in the track grooves 140 of the outer joint member 141, and serve as torque transmission units. In this case, the rollers 144 are externally fitted through interposition of a plurality of rollers 145 arranged on radially outer surfaces of the journal 142 along a circumferential direction.
The outer joint member 141 includes a mouth section 141a and a stem section 141b, which are integrally formed. The mouth section 141a has a cup shape which is opened at one end thereof. The three track grooves 140 extending in the axial direction are formed in a radially inner surface of the mouth section 141a. The tripod member 143 includes a boss 146 and the journal 142. The journal 142 project radially at three equiangular positions in the circumferential direction of the boss.
A female spline 148 is formed in a radially inner surface of the boss 146. An end portion of the shaft 150 on the inboard side is inserted to the boss 146, and a male spline 149 formed in the end portion of the shaft 150 is fitted to the female spline 148 of the boss 146. With this, the shaft 150 and the tripod member 143 are fitted to each other so that torque can be transmitted. A circlip 152 is mounted to the end portion of the shaft 150. With this, the shaft 150 is prevented from coming off.
In the fixed type constant velocity universal joint or the plunging type constant velocity universal joint, in order to prevent leakage of a lubricant such as grease sealed inside the joint and to prevent entry of a foreign matter from outside the joint, in general, a boot 160 made of rubber or resin is mounted between the outer joint member 123 (141) of the constant velocity universal joint and the shaft 130 (150) extending from the inner joint member 126 (143) to close an opening portion of the outer joint member 123 (141) with the boot 160.
As illustrated in FIG. 15 and FIG. 16, the boot 160 includes a large-diameter end portion 160a, a small-diameter end portion 160b, and a bellows portion 160c. The large-diameter end portion 160a is fastened and fixed by a boot clamp 161 to an outer peripheral surface of the opening portion of the outer joint member 123 (141) of the constant velocity universal joint. The small-diameter end portion 160b is fastened and fixed by a boot clamp 162 to an outer peripheral surface of the shaft 130 (150) extending from the inner joint member 126 (143) of the constant velocity universal joint. The bellows portion 160c connects the large-diameter end portion 160a and the small-diameter end portion 160b. The bellows portion 160c has peak portions and root portions which are formed alternately and continuously and is reduced in diameter from the large-diameter end portion 160a to the small-diameter end portion 160b so as to be expandable and contractible.
The constant velocity universal joint has a function to rotate while taking an operation angle. Further, the plunging type constant velocity universal joint has a function to rotate while sliding in the axial direction. Therefore, various proposals have hitherto been made in order to secure durability, such as abrasion resistance and fatigue resistance, of the boot.
Specifically, in order to suppress abrasion in the boot, various measures have been taken in related-art constant velocity universal joints. For example, an inner diameter of the root portions of the boot is increased. As in Patent Literature 1, the inner peripheral surfaces of the root portions of the boot are formed into a shape which achieves high resistance against abrasion. As in Patent Literature 2, the surface roughness of the outer peripheral surface of the shaft is reduced.
Further, as in Patent Literature 3, a component for reducing abrasion or abnormal sound is added to a material of the boot. As in Patent Literature 4, on a surface of the boot made of a diene-based rubber material, a synthetic resin containing polytetrafluoroethylene powder is arranged as a discontinuous coating. According to Patent Literature 4, such discontinuous coating is arranged to stably achieve a low friction property and abrasion resistance in the surface.
Further, according to Patent Literature 5, the bellows portion is divided into a portion closer to a large-diameter mounting part, a portion closer to a small-diameter mounting part, and a center portion, and the rigidities of those parts are differentiated. Specifically, the relationship of the rigidities of the parts is set to be the center portion>the portion closer to the large-diameter mounting part>the portion closer to the small-diameter mounting part. Further, according to Patent Literature 6, the diameter of the mountain portion and the diameter of the root portion are limited.